mass of dry crucible=34.69g

mass of crucible with magnesium=34.68g
mass of crucible with magnesium oxide =34.97g
we did a lab and igot the mass for magnesium to be 0.17g and the mass for magnesium oxide to be 0.28g.
how can i find the moles of magnesium and the number of molecules of magnesium oxide

You may have made a typo but mass Mg from your data is -0.01 which is impossible.

To answer your question,
mols Mg = mass Mg/atomic mass mg.
# molecules MgO (I wonder if you don't want mols of this, too) = (mass MgO/molar mass MgO)* 6.022E23 = ?

To find the moles of magnesium (Mg) and the number of molecules of magnesium oxide (MgO), we will need to use the given masses and molar masses of the elements.

1. Moles of Magnesium (Mg):
The moles of magnesium can be calculated using the mass of magnesium (0.17g) and the molar mass of magnesium (24.31g/mol).
The formula to calculate moles is:
Moles = Mass / Molar Mass

Moles of Mg = 0.17g / 24.31g/mol
Moles of Mg = 0.007 moles (rounded to three decimal places)

So, the moles of magnesium is approximately 0.007 moles.

2. Number of Molecules of Magnesium Oxide (MgO):
To calculate the number of molecules, we need to convert the mass of magnesium oxide into moles and then use Avogadro's number (6.022 × 10^23 molecules/mol).
The molar mass of magnesium oxide (MgO) is 40.31g/mol (24.31g/mol for Mg + 16g/mol for O).

First, calculate the moles of magnesium oxide using the mass (0.28g) and the molar mass (40.31g/mol):
Moles of MgO = 0.28g / 40.31g/mol
Moles of MgO = 0.006942 moles (rounded to six decimal places)

Now, we can use Avogadro's number to find the number of molecules:
Number of Molecules = Moles × Avogadro's number

Number of MgO = 0.006942 moles × (6.022 × 10^23 molecules/mol)
Number of MgO = 4.18 × 10^21 molecules

So, the number of molecules of magnesium oxide is approximately 4.18 × 10^21 molecules.